Silicon tetrachloride

Silicon tetrachloride

Names
IUPAC name Tetrachlorosilane
Other names Silicon tetrachloride Tetrachlorosilane
Identifiers
CAS Number	10026-04-7^Y
3D model (JSmol)	Interactive image
ChemSpider	23201^Y
ECHA InfoCard	100.030.037
EC Number	233-054-0
PubChem CID	24816
RTECS number	VW0525000
UNII	96L75U0BM3^N
UN number	1818
CompTox Dashboard (EPA)	DTXSID0029711
InChI InChI=1S/Cl4Si/c1-5(2,3)4^Y Key: FDNAPBUWERUEDA-UHFFFAOYSA-N^Y InChI=1/Cl4Si/c1-5(2,3)4
SMILES [Si](Cl)(Cl)(Cl)Cl
Properties
Chemical formula	SiCl4
Molar mass	169.90 g/mol
Appearance	Colourless liquid
Density	1.483 g/cm^3
Melting point	−68.74 °C (−91.73 °F; 204.41 K)
Boiling point	57.65 °C (135.77 °F; 330.80 K)
Solubility in water	Reacts to form silica
Solubility	soluble in benzene, toluene, chloroform, ether[1]
Vapor pressure	25.9 kPa at 20 °C
Magnetic susceptibility (χ)	−88.3·10^−6 cm^3/mol
Structure
Crystal structure	Tetrahedral
Coordination geometry	4
Thermochemistry
Std molar entropy (S^⦵298)	240 J·mol^−1·K^−1[2]
Std enthalpy of formation (ΔfH^⦵298)	−687 kJ·mol^−1[2]
Hazards
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335
Precautionary statements	P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, P405, P501
NFPA 704 (fire diamond)	3 0 2 W
Safety data sheet (SDS)	ICSC 0574 MSDS
Related compounds
Other anions	Silicon tetrafluoride Silicon tetrabromide Silicon tetraiodide
Other cations	Carbon tetrachloride Germanium tetrachloride Tin(IV) chloride Titanium tetrachloride
Related chlorosilanes	Chlorosilane Dichlorosilane Trichlorosilane
Supplementary data page
Silicon tetrachloride (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is ^YN ?) Infobox references

Silicon tetrachloride or tetrachlorosilane is the inorganic compound with the formula SiCl₄. It is a colorless volatile liquid that fumes in air. It is used to produce high purity silicon and silica for commercial applications. It is a part of the chlorosilane family.

Preparation

Silicon tetrachloride is prepared by the chlorination of various silicon compounds such as ferrosilicon, silicon carbide, or mixtures of silicon dioxide and carbon. The ferrosilicon route is most common.^[3]

In the laboratory, SiCl₄ can be prepared by treating silicon with chlorine at 600 °C (1,112 °F):^[1]

Si + 2 Cl₂ → SiCl₄

It was first prepared by Jöns Jakob Berzelius in 1823.^[4]

Brine can be contaminated with silica when the production of chlorine is a byproduct of a metal refining process from metal chloride ore. In rare occurrences, the silicon dioxide in silica is converted to silicon tetrachloride when the contaminated brine is electrolyzed.^[5]

Reactions

Hydrolysis and related reactions

Like other chlorosilanes or silanes, silicon tetrachloride reacts readily with water:

SiCl₄ + 2 H₂O → SiO₂ + 4 HCl

In contrast, carbon tetrachloride does not hydrolyze readily. The reaction can be noticed on exposure of the liquid to air, the vapour produces fumes as it reacts with moisture to give a cloud-like aerosol of hydrochloric acid.^[6]

With alcohols it reacts to give orthosilicate esters:

SiCl₄ + 4 ROH → Si(OR)₄ + 4 HCl

Polysilicon chlorides

At higher temperatures homologues of silicon tetrachloride can be prepared by the reaction:

Si + 2 SiCl₄ → Si₃Cl₈

In fact, the chlorination of silicon is accompanied by the formation of hexachlorodisilane Si₂Cl₆. A series of compounds containing up to six silicon atoms in the chain can be separated from the mixture using fractional distillation.^[1]

Reactions with other nucleophiles

Silicon tetrachloride is a classic electrophile in its reactivity.^[7] It forms a variety of organosilicon compounds upon treatment with Grignard reagents and organolithium compounds:

4 RLi + SiCl₄ → R₄Si + 4 LiCl

Reduction with hydride reagents afford silane.

Comparison with other SiX₄ compounds

	SiH4	SiF4	SiCl4	SiBr4	SiI4
b.p. (˚C)[8]	-111.9	-90.3	56.8	155.0	290.0
m.p. (˚C)[8]	-185	-95.0	-68.8	5.0	155.0
Si-X bond length (Å)	>0.74 [9]	1.55	2.02	2.20	2.43
Si-X bond energy (kJ/mol)[10]	384	582	391	310	234

Uses

Silicon tetrachloride is used as an intermediate in the manufacture of polysilicon, a hyper-pure form of silicon,^[3] since it has a boiling point convenient for purification by repeated fractional distillation. It is reduced to trichlorosilane (HSiCl₃) by hydrogen gas in a hydrogenation reactor, and either directly used in the Siemens process or further reduced to silane (SiH₄) and injected into a fluidized bed reactor. Silicon tetrachloride reappears in both these two processes as a by-product and is recycled in the hydrogenation reactor. Vapor phase epitaxy of reducing silicon tetrachloride with hydrogen at approximately 1250 °C was done:

SiCl
₄(g) + 2 H
₂(g) → Si(s) + 4 HCl(g) at 1250°C^[11]

The produced polysilicon is used as wafers in large amounts by the photovoltaic industry for conventional solar cells made of crystalline silicon and also by the semiconductor industry.

Silicon tetrachloride can also be hydrolysed to fumed silica. High purity silicon tetrachloride is used in the manufacture of optical fibres. This grade should be free of hydrogen containing impurities like trichlorosilane. Optical fibres are made using processes like MCVD and OFD where silicon tetrachloride is oxidized to pure silica in the presence of oxygen.

As a feedstock in production of fused silica.

Safety and environmental issues

Pollution from the production of silicon tetrachloride has been reported in China associated with the increased demand for photovoltaic cells that has been stimulated by subsidy programs.^[12] The MSDS notes that one should "avoid all contact! In all cases consult a doctor! (...) inhalation causes sore throat and burning sensation".^[13]

The hydrolysis of silicon tetrachloride with water adds the hazard of abrasive silica to the acidic reaction of hydrochloric acid to the injury from acid, compounding the damage from the corrosive effect of the acid.

See also

• Silicon tetrachloride (data page)

References

1. P. W. Schenk (1963). "Phosphorus(V) fluoride". In G. Brauer (ed.). Handbook of Preparative Inorganic Chemistry, 2nd Ed. Vol. 1. NY, NY: Academic Press. pp. 282–683.
2. Zumdahl, S. S. (2009). Chemical Principles (6th ed.). Houghton Mifflin. p. A22. ISBN 978-0-618-94690-7.
3. Simmler, W. "Silicon Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_001. ISBN 978-3527306732.
4. Berzelius, Jac. (1824). "Undersökning af flusspatssyran och dess märkvärdigaste föreningar" [Examination of hydrofluoric acid and its most significant compounds]. Kongliga Vetenskapsakademiens Nya Handlingar [New Proceedings of the Royal Academy of Sciences]. 3rd series (in Swedish). 12: 46–98. From pp. 57-58: "Då silicium upphettas i en ström ab chlor, tänder det sig och brinner, samt om gasen innehöll atm. luft, lemnar det kiseljord i form af ett ullikt skelett. […] Silicium glödgadt i en ström af iodgas, har icke kunnat fås att dermed förbinda sig." (When silicon is heated in a stream of chlorine, it ignites and burns, as well as if the gas contained atmospheric air, it leaves silica in the form of an odd "skeleton". If the silicon was previously oxidized to some extent, then the siliceous earth also remains. Silicon burns in chlorine with equal slowness, whether it has lost its flammability in air or not. The product of the combustion is condensed and forms a liquid, which, when freed from it, should be colorless. This liquid is quite volatile and easy-flowing; it evaporates in the open air, almost instantly, with the emission of a white smoke and with a residue of siliceous earth. It has a pungent smell, somewhat like cyanide; precipitated in water, it quickly floats up, dissolves for the most part, but leaves a little siliceous earth undissolved; if the quantity of water is small, e.g., a drop of each, then the chlorosilicon floats around and the silica becomes undissolved in an exfoliated, semi-transparent state. This liquid is analogous to the compound of other electronegative substances with chlorine. Reacts like acid with litmus paper, so that, by its volatility, the paper reddens quite a distance from the point of contact. It is the second known example of a compound in which silicon is volatile. At the ordinary temperature of the air, potassium does not act on it; but if it is heated in the gas of chlorosilicon, it ignites and burns, with a residue of silicon-bound potassium. Silicon heated in a stream of iodine gas, could not be made to bond with it.)
5. White, George Clifford (1986). The handbook of chlorination (2nd ed.). New York: Van Nostrand Reinhold. pp. 33–34. ISBN 0-442-29285-6.
6. Clugston, M.; Flemming, R. (2000). Advanced Chemistry. Oxford University Press. p. 342. ISBN 978-0199146338.
7. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
8. Silicon Compounds, Silicon Halides. Collins, W.: Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc, 2001.
9. "What is the bond length of the H-H bond?". Answers.com.
10. Ebsworth, E. A. V. In Volatile Silicon Compounds; Taube, H.; Maddock, A. G.; Inorganic Chemistry; Pergamon Press Book: New York, NY, 1963; Vol. 4.
11. Morgan, D. V.; Board, K. (1991). An Introduction To Semiconductor Microtechnology (2nd ed.). Chichester, West Sussex, England: John Wiley & Sons. p. 23. ISBN 0471924784.
12. "Solar Energy Firms Leave Waste Behind in China". The Washington Post. 9 March 2008.
13. "International Chemical Safety Cards Tetrachlorosilane".